Morris Minor Owners Club

Ah, so thats why its leaking

I expect you already know this but for coolant use 50/50 Antifreeze water mix for corrosion protection. I like to use the water from my dehumidifier. Premixed antifreeze is also available.
Regards John

The rust stain is from the block, etc, not this cover. The corrosion of the cover is from the anti-freeze itself. It must have been in there a while. It turns acidic and will corrode these light alloys.

Well yes and no, I use antifreeze but not in any measured way, remiss I know.
It does always have antifreeze but I think its the original stat cap from 1965, I had to replace the water pump when I first got the car so for similar reasons should expect this.
Anyway it was easy enough to replace with an ex stock item from Tom Roy, and of course a fresh bottle of antifreeze.

Had exactly the same problem with both thermostat cover and water pump , both basically rotten ! The fun part was removing the thermostat cover with loads of penetrating oil , gentle persuasion and patience over a two day period . Loads of posts on this forum about similar issues with thermostat housings . Interestingly what condition was your thermostat in , mine was covered in crud showing a lack of attention on my part .

Ah, I replaced the stat when I bought the car 7 years ago, it didnt occur to me that I should replace the housing at the same time, even though as a matter of course I replaced the water pump too as it and the radiator were old and very much tired out.
Getting the stat cover free then was quite easy as when I put the new stat in the studs got a liberal coating of grease.